This invention relates to semiconductor devices and processes for their manufacture and more specifically relates to reduced mask processes and termination structures for such devices.
Semiconductor devices, for example, fast recovery diodes (xe2x80x9cFREDxe2x80x9ds) are well known and are a hybrid of Schottky diodes and PN diodes. This arrangement produces a lower forward voltage drop at higher current, along with a higher switching speed than is available in only a PN junction diode or only a Schottky diode. In the present manufacture of such FRED devices, a plurality of spaced P diffusions of any desired topology are formed in an N type wafer. A contact layer of aluminum overlies the full upper surface of the silicon, except for a termination area. PN junction diodes are then formed where the aluminum contacts the surface of a P diffusion and a Schottky diode is formed where the aluminum contacts the Nxe2x88x92 silicon surface between spaced P diffusions.
The topology of the P diffusion can be spaced polygonal annuli, stripes, or the like. The periphery of the die is then surrounded by a termination region.
The manufacturing process for such FREDs has been complicated by a need for 3, 4, 5 or 6 mask steps during the processing of the device. These mask steps are used to define the termination pattern, the P diffusion pattern and the final metallization pattern. The use of a large number of mask steps increases the cost of the final device and is a source of device defects.
It would be desirable to provide a manufacturing process for a FRED and its termination which uses fewer mask steps without sacrificing device quality. It is also desirable to be able to provide a novel terminal structure for any semiconductor device which provides increased breakdown voltage without the need for a large number of mask steps.
In accordance with a first aspect of the invention, a FRED device is manufactured with a single mask step. Thus, an N type wafer is provided and a layer of SiO2 (hereinafter silicon dioxide, or oxide), followed by a layer of Si3 N4 (hereinafter silicon nitride or nitride) is formed atop the wafer. A single mask is used to etch openings in the oxide and nitride layers, having the patterns of spaced P type diffusions to be formed in the silicon for both a termination diffusion and for a PN junction. A P type dopant, for example, boron is then implanted through these windows and is driven into the silicon. The oxide overlying the sides of the diffused regions and under the nitride layer is then etched away thus lifting the nitride layer lattice. A contact metal, for example, aluminum, is then deposited on and overlies the full active surface and the termination surface. The metal then contacts the P diffusions in the active area and the silicon between the spaced diffusions in the active area, thereby defining PN junctions and Schottky diodes in parallel with one another.
The wafer is then subject to a backgrind and to back metal evaporation and to a forming gas anneal.
Note that the entire process above for producing the FRED employs only a single mask. No metal mask is used. A novel termination structure may be added to the FRED, using an additional and second mask, which permits a separate contact to the guard ring to enable the use of the device at higher voltages, for example, 1200 volts.
A novel field plate structure for device termination is also provided which is applicable to FREDs as well as other devices. In general, all high voltage semiconductor devices use field plate structures to obtain the highest possible device breakdown voltage for a given termination structure design. The field plate structures do not conduct device currents and hence have negligible impact on other device parameters such as forward voltage drop during device operation. Thus, in general, a thin layer of high resistivity amorphous silicon is deposited on top of the final metallization to evenly distribute the electric field across the termination structure. This results in a stable field termination structure and improves yield. The amorphous silicon is etched away from the pad area by an additional mask step at the end of the process.
However, the amorphous silicon can be left in place and wire bonds to the underlying aluminum contact can be made through the amorphous silicon without added tooling.
Still further, it has been found that the amorphous silicon can be placed below the metal to avoid the pad mask, producing a new type of FRED with the amorphous silicon layer between the Schottky structure and the single crystal silicon with state of art FRED characteristics.
While this termination is very useful with a FRED structure it can be used in any kind of device such as the termination for a power MOSFET or IGBT.
As a still further feature of the invention, palladium metal can be used in place of aluminum to reduce the Irr of the device. More specifically, during the operation of a FRED device, stored charge produced by injected minority carriers from the PN junctions must be removed after turn off. Removal of stored charge determines the switching characteristics of the FRED device, including switching speed and xe2x80x9csoftnessxe2x80x9d. A large stored charge also exerts excessive electrical stress during turn off and should be as low as possible. Consequently, device improvement can be obtained by controlling the injection of majority carriers during operation. A novel palladium Schottky structure is used in place of an aluminum Schottky structure since it will require a different current density to turn on the PN junction because of the lower Schottky barrier height of the palladium Schottky compared to the aluminum Schottky always used in a FRED device. That is, the Schottky contact of the FRED conducts until there is a 0.7 volt drop to cause the PN junction to conduct. It also has a dramatic impact on the stored charge injected in the device during device operation.
More generally, this aspect of the invention uses a lower barrier height material than aluminum for the Schottky portion of a FRED device to control the switching speed, softness and Irr (stored charge) of the device.